linux-kernel - [PATCH 2/2] getrusage: use sig->stats

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Date: Fri, 19 Jan 2024 15:15:29 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Dylan Hatch <dylanbhatch@...gle.com>, Kees Cook <keescook@...omium.org>,
	Frederic Weisbecker <frederic@...nel.org>,
	"Joel Fernandes (Google)" <joel@...lfernandes.org>,
	Ard Biesheuvel <ardb@...nel.org>,
	"Matthew Wilcox (Oracle)" <willy@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Vincent Whitchurch <vincent.whitchurch@...s.com>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	Luis Chamberlain <mcgrof@...nel.org>,
	Mike Christie <michael.christie@...cle.com>,
	David Hildenbrand <david@...hat.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Stefan Roesch <shr@...kernel.io>, Joey Gouly <joey.gouly@....com>,
	Josh Triplett <josh@...htriplett.org>, Helge Deller <deller@....de>,
	Ondrej Mosnacek <omosnace@...hat.com>,
	Florent Revest <revest@...omium.org>,
	Miguel Ojeda <ojeda@...nel.org>, linux-kernel@...r.kernel.org
Subject: [PATCH 2/2] getrusage: use sig->stats_lock

Rather than lock_task_sighand(), sig->stats_lock was specifically designed
for this type of use. This way getrusage runs lockless in the likely case.

TODO:
	- Change do_task_stat() to use sig->stats_lock too, then we can
	  remove spin_lock_irq(siglock) in wait_task_zombie().

	- Turn sig->stats_lock into seqcount_rwlock_t, this way the
	  readers in the slow mode won't exclude each other. See
	  https://lore.kernel.org/all/20230913154907.GA26210@redhat.com/

	- stats_lock has to disable irqs because ->siglock can be taken
	  in irq context, it would be very nice to change __exit_signal()
	  to avoid the siglock->stats_lock dependency.

Signed-off-by: Oleg Nesterov <oleg@...hat.com>
---
 kernel/sys.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/kernel/sys.c b/kernel/sys.c
index 70ad06ad852e..f8e543f1e38a 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -1788,7 +1788,9 @@ void getrusage(struct task_struct *p, int who, struct rusage *r)
 	unsigned long maxrss;
 	struct mm_struct *mm;
 	struct signal_struct *sig = p->signal;
+	unsigned int seq = 0;
 
+retry:
 	memset(r, 0, sizeof(*r));
 	utime = stime = 0;
 	maxrss = 0;
@@ -1800,8 +1802,7 @@ void getrusage(struct task_struct *p, int who, struct rusage *r)
 		goto out_thread;
 	}
 
-	if (!lock_task_sighand(p, &flags))
-		return;
+	flags = read_seqbegin_or_lock_irqsave(&sig->stats_lock, &seq);
 
 	switch (who) {
 	case RUSAGE_BOTH:
@@ -1829,14 +1830,23 @@ void getrusage(struct task_struct *p, int who, struct rusage *r)
 		r->ru_oublock += sig->oublock;
 		if (maxrss < sig->maxrss)
 			maxrss = sig->maxrss;
+
+		rcu_read_lock();
 		__for_each_thread(sig, t)
 			accumulate_thread_rusage(t, r);
+		rcu_read_unlock();
+
 		break;
 
 	default:
 		BUG();
 	}
-	unlock_task_sighand(p, &flags);
+
+	if (need_seqretry(&sig->stats_lock, seq)) {
+		seq = 1;
+		goto retry;
+	}
+	done_seqretry_irqrestore(&sig->stats_lock, seq, flags);
 
 	if (who == RUSAGE_CHILDREN)
 		goto out_children;
-- 
2.25.1.362.g51ebf55